1. Field of the Invention
This invention relates to the coupling of laser energy into a proximal end of an optical fiber array including one or more Optical fibers. More Particularly, this invention relates to launching a soft-focused laser beam into the fiber array at the beam waist.
2. Description of the Related Art
Traditionally, laser energy is coupled into an optical fiber array by passing energy from a laser through one or more lenses. The laser creates a collimated beam of light and the lenses focus the beam to a sharp spot or point. The optical fiber array is placed within the beam a short distance in front of or behind the focal point such that the cross sectional area of the beam substantially matches the cross sectional area of the ends of the fibers.
The amount of energy that can be coupled from a particular laser beam into a particular optical fiber of an array is dependent on the fluence-damage-threshold of the fiber. For optimal coupling, the fluence of the laser beam must be less than the fluence-damage-threshold of the optical fiber. If the fluence of the laser beam is higher than the fluence-damage-threshold of the fiber, the fiber will be burned or destroyed.
In an ideal situation a laser beam distributes a uniform fluence across the incident end of the optical fiber array. Actual laser beams, however, do not distribute a uniform fluence across a cross section of the beam. This results in burnt or destroyed spots at the input end of the fiber array.
Focusing of a laser beam can, for example, be accomplished by a plano-convex lens. The plano-convex lens converges a collimated beam that passes through it into an infinitesimally small spot or focal point. The focal point contains substantially all the energy projected by the laser and can potentially damage the end of the optical fiber should the fiber end be positioned at the focal point. Because of this potential damage, the end of the optical fiber array is generally restricted from placement at the focal point in high energy coupling systems.
U.S. Pat. No. (USP) 4,732,448 to Goldenberg teaches the technique of focusing a laser beam in front of a fiber optic array. Even when the end of the fiber array is positioned away from the focal point of a focused laser beam, laser energy may be unevenly distributed across the input area of the optical fiber. In other words, the result can be nonuniform transmission of laser energy through different cross sectional parts of the optical fiber array.
Furthermore, laser beams, such as those from transverse electric discharge lasers like excimer lasers, are rectangular in cross sectional shape and not conducive to being coupled to a traditionally circular or hexagonal fiber array configuration. Coupling of the rectangular laser beam into the non-rectangular array will result in energy loss due to overspill of the beam beyond the optical fiber array.
Various apparatus and methods have been attempted to solve the above efficiency draining problems related to the coupling of laser energy into an optical fiber array's proximal end, the mismatching of the beam fluence levels to the fluence-damage-threshold of the fiber, the uneven distribution of laser energy over a cross section of a fiber, and the difficulty of matching a rectangular shaped laser beam to a circular or hexagonal optical fiber array configuration.
An apparatus for homogenization of laser beam fluence levels at the incident end of an optical fiber can permit laser energy-coupling with less damage to an optical fiber. Such an apparatus and method can increase the amount of laser energy coupled to an optical fiber without the optical fiber being subjected to fluence levels greater than the damage threshold of the fiber.
The problem of uneven laser energy distribution across the cross section of an optical fiber array end has been remedied in various ways. These remedies include beam homogenizers, aspheric lenses, afocal doublets, apparatus for focusing a laser into a focal line in order to couple the line into fibers arranged in a linear configuration, and an apparatus using a meniscus lens containing a spherical aberration.
A beam homogenizer folds the side lobes of an incident laser beam back into the central portion of the beam creating a homogeneous beam of uniform intensity across its energy profile. U.S. Pat. No. 4,793,694 to Liu teaches a method of homogenizing a laser beam using symmetrical mirror pairs located along the axis of the laser beam. The first set of mirrors separates the side lobes of the beam from the center of the beam and the second set of mirrors folds the beam imaging each side lobe to the opposite side of the central axis from where it originated. The resultant-superimposed beam is more uniformly distributed across its cross section than the original beam. The patent teaches that a trapezoidal prism may also be employed. See also, Roy J. Bruno et al., "Laser Beam Shaping for Maximum Uniformity and Minimum Loss", Lasers & Applications, April 1987, pp. 91-94.
Aspheric lenses are also used to homogenize a beam cross section or profile and are described in D. Shafer, Gaussian to Flat-Top Intensity Distributing Lens, Optics and Laser Technology, June 1982, pg. 159. An aspheric plate, usually having a conic surface of revolution about the lens axis, modifies an incoming laser beam so that the intensity of the beam becomes uniform across the beam profile. A second plate modifies the light waves eliminating spherical aberration without modifying the uniformly distributed energy profile. The result is an unaberrated substantially uniform distribution of laser energy across the profile of the beam.
Afocal doublets, another known method of homogenizing the energy distribution of a laser beam, comprise closely spaced positive and negative lens focusing elements. D. Shafer, Gaussian to Flat-Top Intensity Distributing Lens, Optics and Laser Technology, June 1982, pg. 159. In a typical arrangement, the lenses are bent to introduce large amounts of spherical aberration. A laser beam is passed through the first element. Paraxial rays remain essentially unaffected, but other rays will encounter a great amount of uncorrected spherical aberration. The paraxial rays remain collimated in space between the first and second elements, but the rays that encountered the spherical aberration converge between the lens elements. When the rays reach the second lens, the converging rays are contained in a smaller area increasing the fluence of the periphery of the beam. Meanwhile, the energy of the centrally located collimated rays remains unchanged. The second element of the doublet has an equal and opposite spherical aberration with respect to the first element. The second element recollimates the energy beam such that the result is a collimated substantially uniform energy distribution across the beam profile.
An apparatus to focus a laser beam into a focal line then couple the line into fibers arranged in a linear array is taught in U.S. Pat. No. 5,016,964 to Donnelly. The focusing of the laser into a finite line homogenizes the beam such that fluence 37 spikes" do not damage the individual fibers arranged linearly.
Finally, U.S. Pat. No. 4,998,794 to Holzman teaches another technique of homogenizing the fluence of a laser beam at the incident end of an optical fiber array which includes the use of a meniscus lens. A spherical aberration located in the central area of the lens blurs the focal point and reduces the peak fluence levels at the incident end of the optical fiber array. The focal point formed by the meniscus lens consists of laser energy originating from the outer portions of the beam profile folded into the inner portion of the profile. This creates a uniform distribution of energy within the focal spot. It should be noted that the power coupling efficiency is reduced due to the aberration at the center of the meniscus lens.
All the above methods for alleviating the problem of uneven energy distribution across a beam profile requires the laser beam energy to be focused either to a focal point, line, or a blurred focal point. With the focal point or focal line, the beam is focused either just in front of or just behind the proximal end of the fibers. A maximum homogeneous energy fluence transfer from beam to fiber is difficult to acquire in this configuration because of the geometry of the light rays. Furthermore, if the beam is focused to a blurred focal point, the power coupling efficiency is reduced. In conclusion, it should be noted that maximum laser fluence energy coupling is difficult to achieve due to alignment and maintenance requirements of all the above mentioned laser, lens, and optical fiber end configurations.
In all the techniques, except the apparatus that focuses laser energy into a line, there is the additional problem of manipulating the naturally rectangular shaped laser beam into a circular or hexagonal shaped beam to accommodate a circular or hexagonal optical fiber array. One method used for adjusting the shape of the laser beam is to pass the beam through a spatial filter prior to its impingement upon the optical fiber. The spatial filter rounds or shapes the beam by blocking the peripheral portions of the rectangular beam. Unfortunately, this technique results in significant beam energy loss prior to coupling with the optical fiber end. Thus, a coupling system is desired wherein the shape of the fiber optic cable closely matches the rectangular shape of the laser beam.
A known method for intentionally limiting the energy of a laser beam prior to impingement upon an optical fiber array includes the use of an intensity adjusting device. An intensity adjusting device blocks predetermined portions of the laser beam in order to decrease its total energy. If an intensity adjustment device is used to limit laser energy in any of the above described methods and apparatus, a silhouette of the device may be focused onto the proximal end of the optical fiber array resulting in further nonuniform fluence across the proximal surface of the fiber end. Thus, there is a need for a method and apparatus which can vary a laser beam's total energy prior to the beam's impingement on the proximal end of an optical fiber array by using an intensity adjustment device, and still have a homogeneous laser energy fluence across the cross section of the optical fiber array's end.
U.S. Pat. No. 5,069,527 to Johnson, Jr. and 5,100,231 to Sasnett et al. both teach an apparatus that uses a laser beam waist, but make no mention of using the beam waist to focus laser energy homogeneously into a fiber optic array. Instead, Johnson Jr. and Sasnett teach how to find a laser beam waist and measure its mode quality.
The following patents are related to the use of a laser apparatus having a concave reflector: U.S. Pat. No. 4,942,588; 4,723,247; 5,101,415; 4,884,281; 4,554,666 and 4,151,487. Furthermore, the following U.S. patents discuss laser cavity design and laser beam optical configurations: 5,084,884; 5,079,445; 5,078,491; 5,070,505; 5,064,284; 5,063,569; 5,042,047; 4,942,586, and 4,426,707.